As a typical ammonia synthesis method, the Haber-Bosch method uses doubly promoted iron catalyst containing several percent by mass of Al2O3 and K2O in Fe3O4 as a catalyst, and brings a mixed gas of nitrogen and hydrogen into contact with the catalyst under high temperature and high pressure conditions to produce ammonia. This technology is widely used industrially in the production process as almost the same as it was completed.
On the other hand, a method of producing ammonia at a temperature lower than the reaction temperature of the Haber-Bosch method has been studied. Catalysts capable of producing ammonia by contacting with nitrogen and hydrogen have been investigated, and transition metals have been studied as their catalytically active components. Among them, a method using ruthenium (Ru) as a catalyst active component on various catalyst supports and using it as a catalyst for ammonia synthesis has been proposed as an efficient method (for example, Patent Document 1).
It is known that a catalyst using a transition metal such as Ru has very high activity and ammonia can be produced under milder conditions than the reaction conditions used in the Harbor-Bosch method. For example, it is known that the reaction proceeds even at a low temperature in the range of the reaction temperature 200-400° C. and under a low pressure in the range from atmospheric pressure to about 1.1 MPa.
A calcium aluminosilicate constituted by CaO, Al2O3, and SiO2, which has the same type crystal structure as Mayenite is called a “Mayenite type compound”. The representative composition of the mayenite type compound is represented by 12CaO.7Al2O3 and the mayenite type compound has a structure in which two oxygen atoms are included as “free oxygen” in a space of a cage formed by its crystal skeleton.
The present inventors have found that a catalyst in which a transition metal is supported as a catalytically active component on a Mayenite compound in which free oxygens in the Mayenite type compound are substituted with electrons (hereinafter referred to as C12A7 electride) has a high activity as a catalyst for ammonia synthesis. (Patent Document 2, Non-Patent Document 1).
Furthermore, the present inventors have found that a supported metal catalyst using a compound such as a metal amide compound has high activity as a catalyst for ammonia synthesis. (Patent Documents 3 and 4).
These catalysts have sufficient reactive activity even under a reaction condition of lower temperature and lower pressure than the reaction condition of the Haber-Bosch method.
On the other hand, various metal hydrides are known and used for various purposes (Non-Patent Document 2). The metal hydrides are usually obtained by heating a metal in a hydrogen atmosphere.
For example, an alkaline earth metal hydride such as CaH2 is used as a drying agent or a reducing agent for a solvent because it reacts with water to generate hydrogen. Since a rare earth metal hydride such as LaH2 absorbs more hydrogen atoms into its molecule and becomes an ultrahigh concentration hydride, so the rare earth metal hydride is used as a hydrogen storage and/or release material. The metal hydride is also used as a material for manufacturing a nitride phosphor, an electron-emitting electrode, and the like.    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2006-231229    Patent Document 2: WO 2012/077658    Patent Document 3: WO 2014/034473    Patent Document 4: WO 2016/088896    Non-Patent Document 1: Kitano, M., Inoue, Y., Yamazaki, Y., Hayashi, F., Kanbara, S., Matsuishi, S., Yokoyama, T., Kim, S. W., Hara, M., Hosono, H., “Nature Chemistry” 4, 934-940 (2012).    Non-Patent Document 2: W. G. Bos, K. H. Gayer, “Journal of Nuclear Materials”, Vol. 18, Issue 1, p. 1-30 (1966).